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Abstract: : Metalloporphyrins have been shown to bind axial ligands in a variety of environments including the vacuum/solid and solution/solid interfaces. Understanding the dynamics of such interactions is a desideratum for the design and implementation of next generation molecular devices which draw inspiration from biological systems to accomplish diverse tasks such as molecular sensing, electron transport, and catalysis to name a few. In this article, we review the current literature of axial ligand coordination to surface-supported porphyrin receptors. We will focus on the coordination process as monitored by scanning tunneling microscopy (STM) that can yield qualitative and quantitative information on the dynamics and binding affinity at the single molecule level. In particular, we will address the role of the substrate and intermolecular interactions in influencing cooperative effects (positive or negative) in the binding affinity of adjacent molecules based on experimental evidence and theoretical calculations. 

We present a quantitative study comparing the binding of 4-methoxypyridine, MeOPy, ligand to Co( ii )octaethylporphyrin, CoOEP, at the phenyloctane/HOPG interface and in toluene solution. Scanning tunneling microscopy (STM) was used to study the ligand binding to the porphyrin receptors adsorbed on graphite. Electronic spectroscopy was employed for examining this process in fluid solution. The on surface coordination reaction was completely reversible and followed a simple Langmuir adsorption isotherm. Ligand affinities (or Δ G ) for the binding processes in the two different chemical environments were determined from the respective equilibrium constants. The free energy value of −13.0 ± 0.3 kJ mol −1 for the ligation reaction of MeOPy to CoOEP at the solution/HOPG interface is less negative than the Δ G for cobalt porphyrin complexed to the ligand in solution, −16.8 ± 0.2 kJ mol −1 . This result indicates that the MeOPy–CoOEP complex is more stable in solution than on the surface. Additional thermodynamic values for the formation of the surface ligated species (Δ H c = −50 kJ mol −1 and Δ S c = −120 J mol −1 ) were extracted from temperature dependent STM measurements. Density functional computational methods were also employed to explore the energetics of both the solution and surface reactions. At high concentrations of MeOPy the monolayer was observed to be stripped from the surface. Computational results indicate that this is not because of a reduction in adsorption energy of the MeOPy–CoOEP complex. Nearest neighbor analysis of the MeOPy–CoOEP in the STM images revealed positive cooperative ligand binding behavior. Our studies bring new insights to the general principles of affinity and cooperativity in the ligand–receptor interactions at the solution/solid interface. Future applications of STM will pave the way for new strategies designing highly functional multisite receptor systems for sensing, catalysis, and pharmacological applications. 

STM can effectively probe single porphyrin receptor-ligand binding events at the solution/solid interface and provide both qualitative and quantitative information about molecule binding affinity, reaction kinetics and thermodynamics. 

Porphyrins are fascinating molecules with applications spanning various scientific fields. In this review we present the use of periodic density functional theory (PDFT) calculations to study the structure, electronic properties, and reactivity of porphyrins on ordered two dimensional surfaces and in the formation of nanostructures. The focus of the review is to describe the application of PDFT calculations for bridging the gaps in experimental studies on porphyrin nanostructures and self-assembly on 2D surfaces. A survey of different DFT functionals used to study the porphyrin-based system as well as their advantages and disadvantages in studying these systems is presented. 

One of the common practices in the literature of molecular desorption is the comparison of theoretically (mostly using DFT) calculated single molecule adsorption energies with experimental desorption energies from studies like temperature programmed desorption (TPD) etc. Comparisons like those do not consider that the experimental desorption energies are obtained via ensemble techniques while theoretical values are calculated at the single molecule level. Theoretical values are generally based upon desorption of a single molecule from a clean surface, or upon desorption of an entire monolayer. On the other hand, coverage dependent molecule–molecule interactions add to and modify molecule–substrate interactions that contribute to the experimentally determined desorption energies. In this work, we explore the suitability of an additive nearest neighbor model for determining general coverage dependent single molecule desorption energies in non-covalent self-assembled monolayers (SAMs). These coverage dependent values serve as essential input to any model attempting to reproduce coverage dependent desorption or for understanding the time dependent desorption from a partially covered surface. This method is tested using a case study of coronene adsorbed on Au(111) and HOPG substrates with periodic DFT calculations. Calculations show that coronene exhibits coverage and substrate dependence in molecular desorption. We found that intermolecular contact energies in the coronene monolayer are not strongly influenced by the HOPG substrate, while coronene desorption on Au(111) exhibits strong cooperativity where the additive model fails.